Academic literature has taught that by controlling the pH of fermentations at or near the optimum pH for the primary fermentation microorganism, the concentration of the desired fermentation product is enhanced. For instance, by fixing the pH of a corn mash fermentation at 5.0-5.5, which is optimum for yeast, the yield of ethanol is enhanced. Use of strong alkalis, such as NaOH or KOH, can result in pH swings high or low that can cause a significant reduction in fermentation performance.
The fermentation process for converting an organic substrate into simpler organic chemicals, such as the conversion of glucose in corn mash into ethanol and carbon dioxide, is accomplished through microbiological activity. Microorganisms perform the functions of fermentation optimally under a narrow window of temperature, pH, and nutrient conditions. There are many types of industrial fermentation processes, such as fuel ethanol production, sewage digestion, the making of bread, beer, wine, and cheese, and pharmaceutical applications to produce important chemicals, such as vancomycin, paclitaxel, insulin lispro. Each industrial fermentation requires different conditions for optimum yield of desired products.
When a particular microorganism is introduced into a selected growth medium, growth in the population of the microorganism does not occur immediately. This period of adaptation is called the lag phase. The lag phase is followed by the exponential growth phase, where the rate of growth of the microorganism steadily increases. Toward the end of the exponential growth phase, the rate of growth slows down, due to the continuously falling concentrations of nutrients and/or continuously increasing concentrations of by-products that are toxic to the microorganism. During this deceleration phase the rate of growth is slowed and then ceases. Unless other microorganisms contaminate the culture, the biomass remains constant and the chemical constitution remains unchanged.
It is a goal of industrial fermentations to increase the exponential growth rate or to prolong the time during which the microorganisms are performing at an exponential growth rate. In so doing, the final yield of a desired fermentation product will be increased. For instance, in the fuel ethanol industry, the ability to provide a consistent measurable increase in the final percent ethanol concentration from corn mash fermentations may translate into a significant increase in the profitability of the overall process.
A current challenge in the fuel ethanol industry is the build up of contaminants that are inhibitory to the performance of the yeast. The increase in the concentration of acetic acid and lactic acid from inhibitory bacteria present in the corn mash fermentation result in a decrease in the yeast (Saccharomyces cerevisiae) growth rate, resulting in decreased ethanol production. Lactobacillus and Acetobacter are commonly occurring bacterial contaminants in ethanol fermentations that produce lactic acid and acetic acid inhibitory by-products. A study by Ingledew in 2001 (J. Ind. Microbiol. Biotechnol. 2001 March; 26(3): 171-7) showed that the specific growth rates of certain Saccharomyces cerevisiae strains decreased exponentially with the increase of acetic acid or lactic acid concentrations in minimal media at 30° C. Furthermore, the lag phase increase displayed in the growth curves increased exponentially with the increasing acetic acid or lactic acid concentrations. However, a very recent study by Narendranath in 2006 (J. Ind. Microbiol. Biotechnol. 2006 June; 33(6): 469-74) concluded the ethanol production experienced inhibitory effects attributed lactic acid and/or acetic acid concentrations in corn mash fermentation broths could be mitigated when a pH range of approximately 5.0-5.5 was attained in the corn mash. The inhibitory effects were also mitigated in other fermentation solutions, for example, laboratory media. In this study, fermentations were performed at fixed pH values of 4.0, 4.5, 5.0, and 5.5, with dramatic inhibitory effects being observed at pH 4.0 and mild inhibitory effects at pH 5.5. The method for fixing the pH of these laboratory corn mash fermentations was to adjust the pH using 8 M potassium hydroxide (KOH), a strong base, and concentrated sulfuric acid (H2SO4), a strong acid.
For industrial fermentation purposes, the use of a strong acid or a strong base to maintain the pH of a selected growth medium would require very stringent chemical feed control. A slight overdose of either the strong acid or strong alkali into a fermenter could result in a significant departure from the desired pH range for microorganism activity, which would decrease the yield of the desired fermentation product. A significant overdose of a strong acid or base into a fermenter could cause a complete stoppage of the fermentation process altogether.